The invention relates to a through-the-lens alignment system for use in lithography. In particular, it relates to a system which corrects for alignment errors occurring in the optical system due to the effect of environmental changes, such as change in temperature or atmospheric pressure.
One problem in lithography is knowing the position relationship between the camera image and the substrate position.
Present lithographic alignment systems achieve alignment by referencing a fiducial in the reticle to a position of the stage. This assumes that the camera remains stable after the alignment and doesn""t drift due to the effect of environmental conditions on the lens system; and, as a result, it provides no assurance that the image doesn""t move between alignments, creating a misalignment.
In the present invention, alignment is through the lens, with fiducials in the reticle being projected through the lens system and being aligned with sensors carried by the lens and using a reflected image plane, not the image plane on the stage or substrate.
This invention is directed to correcting misalignment errors resulting from changes in the lens system itself. These errors can arise from factors such as temperature change or change in atmospheric pressure.
A fiducial on the reticle adjacent to its working area is projected through the lens, using a wavelength of light that is compatible with the lens. A metrology plate carried by the lens itself has fold mirrors to receive the resultant image and reflect it to detectors in a reflected image plane. This provides feedback to the reticle alignment system. Thus, if environmental conditions associated with the lens create a misalignment due to camera problems, the extent of the change is detected and continuously corrected.
Since the misalignment is not related to the stage, and is not detected at the stage, it is not corrected by moving the stage. Rather, correction is made by moving the reticle until alignment is achieved and detected. This motion is achieved by having a reticle stage with linear motors to move the reticle until alignment is achieved.